1. Technical Field
The present invention relates to a process for production of nitrided steel having high pitting corrosion resistance and superior fatigue strength.
2. Background Art
In recent years, CTV (Continuously Variable Transmission) is widely used as a stepless transmission for automobiles. The CTV is formed by annularly connecting plural pushing blocks with a metallic hoop. High fatigue strength is required for steel used in such hoops and springs because bending force is applied repeatedly. As a technique to improve fatigue strength of various kinds of steel, a nitriding method is known as is disclosed in, for example, Japanese Unexamined Patent Application Publications Nos. 1-142022, 2000-219956, or 2001-26857. However, since the surface is activated during the nitriding process, part of the surface is corroded and pitting corrosion is formed due to the presence of halogens, and corrosion resistance may be deteriorated. Such pitting corrosion often grows in a depth direction, and it is difficult to discover by visual inspection. In particular, in the case of a thin material such as the hoop mentioned above, pitting corrosion causes great deterioration in fatigue strength.
Many such halogens are the chlorine of NaCl. Fine NaCl particles which come from the sea or the human body are present in ordinary environments. FIG. 1 is a SEM photograph showing an example of a particle attached to a steel in an ordinary work environment. This was analyzed by an EDX (Energy Dispersive X-ray Analyzer), and it turned out to be NaCl. Such fine NaCl particles are floating in a production process and an assembling process of parts unless the processes are performed in a clean room. Furthermore, in the case in which fine defects or fine particles are on the surface of the parts, water vapor in the air may easily condense at the points by capillary action (see “Examples of corrosion and kinds of measures to corrosion of metal”, p. 186, published by TECHNOSYSTEM). NaCl particles and water vapor floating in air are contacted on the surface of steel, generating drops of aqueous NaCl, and pitting corrosion may occur by the reaction shown in FIG. 2. In this way, surface pitting corrosion may occur without being in a special corrosion environment, and this pitting corrosion causes great deterioration in fatigue strength.
Conventionally, to prevent pitting corrosion, environmental measures or a method to improve pitting corrosion resistance of a steel itself has been performed. As the environmental measures, contact with water vapor or halogens is prevented. These measures can be performed when parts are produced and assembled in a clean room, although it is difficult to perform in all processes, and it is almost impossible to perform completely. Therefore, pitting corrosion resistance of steel itself is required to be improved.
To improve pitting corrosion resistance, it is effective to passivate the surface. Passivation of the surface by an alloy element control or other passivation treatment of the surface can be performed as the method. In particular, it is extremely efficient to add, for example, Cr as the alloy element control. However, in the case of a steel in which Cr cannot be added, for example, in the case of maraging steel, addition of Cr causes deterioration of strength characteristics. As a surface passivating treatment, a treatment in which steel is immersed in, for example, dichromate solution or nitrite solution, can be performed. However, immersing and drying processes are then required. Furthermore, it is difficult to perform uniform passivation unless the drying process is devised, and there is a case in which the steel becomes rusty.
It is known that thin passivated layer having a thickness of not more than 10 nm and containing FeOOH, Fe3O4, or Fe2O3 is formed on the surface of the iron (see “Corrosion and Corrosionproofing Handbook”, 2000, p. 23, published by MARUZEN). An important fact about pitting corrosion resistance is to form passivated layer as uniformly as possible. Partial passivated layer and partial oxide layer cause forming of a local battery and the pitting corrosion resistance may be deteriorated.